1. Field of the Invention
The invention relates to a digital communications system in which data is to be transmitted over a frequency or time dispersive medium in which fading of the transmitted signal is caused by multipath or other type of nonlinear distortion.
2. Description of the Prior Art
Digital communications systems wherein transmission is made through a frequency or time dispersive medium have used many different modulation types. Various systems have employed pulse duration modulation, frequency shift keying and phase shift keying including quarternary phase shift keying as well as numerous other schemes. In each of these systems, a modulator has operated directly upon a locally generated sine wave carrier signal. The bandwidth of the transmitted signal in previous systems was dependent therefore only upon the spectral spreading caused by the modulation of the sine wave carrier signal. Unfortunately, the bandwidth of a frequency selected fade moving in frequency through a transmission band was frequently equal or greater than the bandwidth of the transmitted signal. When the fade was in time alignment with the transmitted signal, the signal would be completely obliterated before it could reach the receiver.
The systems of the prior art show several methods for attempting to overcome the frequency selective fading problem. Many systems employed a plurality of receiving antennas spaced apart from one another so that the signal received by at least one of the antennas would not be affected by the selected fade. As the frequency selective fade could obliterate the entire signal over a relatively large region of, such systems could not entirely solve the problem. Hence, the only truly effective remedy was to substantially lengthen the transmission time for each bit of digital data to increase the probability that the fade would not be present during the entire bit transmission time. This, of course, reduced the maximum bit transmission rate for the entire digital communications system. Moreover, the average transmitter power was also reduced in many systems as the power available for each pulse transmission was spread out over a longer time interval.
Moreover, in many of the systems of the prior art, efficient use was not made of the entire bandwidth allocated for each digital transmission channel. As the bandwidth was dependent only upon the modulation present, it was common that only a small portion of the entire bandwidth was used at any one transmission interval.